The present invention relates to problems extant in printing, particularly with respect to offset printing.
During operation of an offset printing press, an ink pattern is applied to sheet material at a printing station by a blanket that is mounted to a blanket cylinder. The pattern is applied to the blanket by a plate cylinder. For full color printing, lithographic printing typically makes use of four such blankets and cylinders at four printing stations: one for each process color, and one for providing black. Full color images are formed by superimposing the four patterns onto one another. In order for the four images laid down by the four separate blankets to properly form an image, the images formed at each printing station by each blanket must be in registration with respect to each other.
Virtually anything that distorts the geometry of the web (typically paper) on which the image is formed in ways that are not anticipated and corrected for will cause the registration of images to be inexact. The less exact the registration of images, the greater the composite image will be seen to blur.
One source of distortion arises from the tendency of the web to absorb moisture as it traverses the printing stations. The source of this moisture is in the fountain solution that is applied to the plate that is used to mask off those portions of the plate on which one does not want ink (which is hydrophobic) to appear. The web may absorb this moisture in sufficient quantities for the web to experience "fan-out," which is a form of lateral expansion. As this can occur in each printing station and can progressively worsen from station to station, the lateral geometry of the web may become increasingly widened and distorted as the web passes through the press. This lateral distortion results in the misregistration of images alluded to above.
A common solution to this problem is to create a compensatory distortion in the web before it arrives at the blanket portion of the printing station. So-called "bustle wheels" have been used for this purpose. The bustle wheel is a rotating disk that presses a width-contracting furrow into the web prior to the printing station. Hence, after the web has expanded due to moisture absorption, its expansion is compensated for by the contraction imposed by the bustle wheel, and registration is enhanced. The bustle wheel can be viewed as a solution to a very specific type of distortion that arises downstream in the printing units.
Another common source of web distortion is the tendency of the web to wrinkle as it is used in lithographic printing. Therefore, significant attention has been paid to this type of distortion. The sources of wrinkling are varied, and include vibrations and mechanical misalignments, as well as inconsistencies in the production of the web itself. One solution to the problem of wrinkles is attempting to eliminate them after they have formed. For example, a concave spreader roller is often employed to laterally spread the wrinkled web, thereby ridding the web of wrinkles that have already formed. The web is general wrapped about a 90-180 degree arc of a rigid steel roller that is concave in profile. The movement of the web in cooperation with so great an extent of the concave metal roll causes the web to be laterally stretched. A variant of the concave spreader roller is a cylindrical roll that is revolved about a curved axis of rotation. Unfortunately, a spreader roller cannot be placed in between the printing units, as the passage of an even partially inked web over a portion of a curved roller would cause the image to smear. Similarly, it is impractical to utilize a bustle wheel downstream of a printing station, as this too would smear the ink on the web if not limited to use in a circumferentially non-print area. Spreader rollers are thus similar to bustle wheels in that each is limited to addressing its own particular problem upstream from the printing unit.
Furthermore, neither of these approaches squarely addresses what occurs at the printing station itself, where many wrinkles actually form. They either anticipate the formation of a distortion and attempt to provided a fan out counter measure (e.g., the bustle wheel), or they attempt to resolve the problem of wrinkling after it has occurred but before the web arrives at the printing stations (the spreader roller).
Wrinkles may form at the nip of the blanket cylinder where the web moves past the blanket cylinder as the result of an irregular velocity profile across the nip of the blanket cylinder. Due to the geometric distortion that arises when a web passes over a cylinder and a blanket that are each somewhat deformable, the effective velocity with which the web is propelled past the nip of the blanket cylinder varies along the linear extent of the nip itself. For example, the effect of cylinder bending and/or blanket stiffness and/or compressibility may cause the web passing through the nip to have a varying velocity profile in which the center of the web moves faster than the periphery of the web, which gives rise to wrinkling. This is an example of a problem arising at a printing station that would best be solved at the printing station and that which would not smear the image.
There remains a need for a mechanism for smoothening out wrinkles at the blanket cylinder within the printing station that can be used in each printing station, thereby assuring proper registration of images from printing station to printing station.